Bacteria called Helicobacter Pylori (HP) are generally known which cause gastric ulcers and gastritis.
If HP is present in the stomach of a patient, an antibiotic should be administered to the patient for bacteria removal treatment. Therefore, it is indispensable to check if the patient has HP. HP has a high urease activity for decomposing urea into carbon dioxide and ammonia.
Carbon has isotopes having mass numbers of 12, 13 and 14, among which the isotope 13C having a mass number of 13 is easy to handle because of its non-radioactivity and stability.
If the concentration of 13CO2 as a final metabolic product in breath of the patient, more specifically, a 13CO2/12CO2 concentration ratio, can successfully be determined after 13C-labeled urea is administered to the patient, the presence of HP can be confirmed.
However, the 13CO2/12CO2 concentration ratio in naturally occurring carbon dioxide is 1:100, making it difficult to accurately determine the concentration ratio in the breath of the patient.
There have conventionally been known methods for determining a 13CO2/12CO2 concentration ratio by way of infrared spectrophotometry (see Japanes Examined Patent Publication No. 61-42219 (1986) and No. 61-42220 (1986)).
The method disclosed in Japanese Examined Patent Publication No. 61-42220 employs two cells respectively having a long path and a short path. The path lengths of the cells are adjusted so that a 13CO2 absorbance in one of the cells is equalized with a 12CO2 absorbance in the other cell. Light beams respectively having wavelengths suitable for determination of the 13CO2 absorbance and the 12CO2 absorbance are applied to the respective cells, and the intensities of transmitted light beams are measured. According to this method, an absorbance ratio for the concentration ratio in naturally occurring carbon dioxide can be set at 1. Therefore, the absorbance ratio is changed correspondingly to a change in the concentration ratio. This allows for detection of the change in the concentration ratio.
(A) Even if the methods employing the infrared spectrophotometry are used, it is difficult to detect a slight change in the concentration ratio. The sensitivity can be enhanced by using longer cells, but the use of the longer cells increases the size of the isotopic gas analyzer.
Another approach is to provide mirrors at opposite ends of the cells for reflecting the light beams many times. However, the cells each have a greater volume, so that the isotopic gas analyzer has a correspondingly greater size.
It is therefore an object of the present invention to provide a stable isotope measurement method, which can determine the concentrations of component gases with a satisfactory measurement reproducibility and with a higher measurement accuracy by introducing a gas specimen containing carbon dioxide 13CO2 and carbon dioxide 12CO2 as the component gases into cells, measuring the intensities of light beams transmitted through the cells at wavelengths suitable for analysis of the respective component gases, and processing data indicative of the light intensities, and yet is free from a size increase.
(B) In the methods employing the infrared spectrophotometry, a reference gas having a CO2 concentration of zero, i.e., air having passed through a carbon dioxide absorbent, is filled in the cells, and a reference absorbance measuring process is preliminarily performed for accurate measurement of the absorbances of 12CO2 and 13CO2.
Where the carbon dioxide absorbent is used as described above, the carbon dioxide absorbent is gradually deteriorated, and it is difficult to determine when the absorbent needs replacement.
The replacement time may be indicated on the basis of the number of times of the analysis, or determined on the basis of a change in the color of the carbon dioxide absorbent which is adapted to be colored by a reaction with carbon dioxide.
Where the determination of the replacement time is based on the number of the times of the analysis, however, the analysis may suffer from an error which occurs due to variations in the absorption capacity of the carbon dioxide absorbent depending on production lots.
Where the carbon dioxide absorbent variable in color is used, the color of the absorbent returns to its original color when the air flow is stopped. Therefore, it is difficult to determine the replacement time.
It is therefore another object of the present invention to provide a method of judging the absorption capacity of a carbon dioxide absorbent, which can accurately indicate a replacement time of the carbon dioxide absorbent by quantizing the degree of the deterioration of the carbon dioxide absorbent.